Propulsion device for ship and ship having the same

ABSTRACT

Disclosed are a propulsion device for a ship and a ship having same. The propulsion device, according to an embodiment of the present invention, comprises: a rear propeller fixed to a first driveshaft; a front propeller positioned in front of the rear propeller and supported rotably on the first driveshaft; a counter rotation device, which penetrates the hull, and which counter-rotates the front propeller and the rear propeller on the basis of the rotation of a second driveshaft, which is installed perpendicular to the first driveshaft; and a housing installed so as to envelope the second driveshaft and the counter rotation device.

TECHNICAL FIELD

Embodiments of the present invention relate to a ship propulsion device and a ship having the same, and more particularly to a ship propulsion device in which two propellers generate propulsive force via counter rotation thereof and a ship having the same.

BACKGROUND ART

Ships have a propulsion device to generate propulsive force for sailing. In general, a single propeller is used in the propulsion device. However, the propulsion device having a single propeller cannot acquire propulsive force from rotational energy of water streams, and thus causes substantial energy loss.

A Counter Rotating Propeller (CRP) type propulsion device is a device that acquires propulsive force from rotational energy without energy loss. In the counter rotating propeller type propulsion device, two propellers installed on the same axis generate propulsive force via counter rotation thereof. A rear propeller of the counter rotating propeller type propulsion device is rotated in reverse with respect to a rotating direction of a front propeller, thereby acquiring propulsive force from rotational energy of fluid caused by the front propeller. Accordingly, the counter rotating propeller type propulsion device may exhibit higher propulsion performance than the aforementioned propulsion device having a single propeller.

The counter rotating propeller type propulsion device includes an inner shaft connected to an engine within a hull, a rear propeller coupled to a rear end of the inner shaft, a hollow outer shaft rotatably installed around an outer surface of the inner shaft, and a front propeller coupled to a rear end of the outer shaft. In addition, the counter rotating propeller type propulsion device includes a counter rotation unit installed within the hull to reverse rotation of the inner shaft and transmit reversed rotation to the outer shaft. A typical planetary gear mechanism is used as the counter rotation unit.

However, in the case of the above-described counter rotating propeller type propulsion device, the hollow outer shaft has difficulty in center alignment with respect to the inner shaft upon installation of the counter rotating propeller type propulsion device to a ship. In addition, the outer shaft needs an increased lubrication area for reduction in friction between the inner shaft and the outer shaft. The counter rotation of the inner shaft and the outer shaft causes shear of a lubrication layer between the inner shaft and the outer shaft, which makes it difficult to realize efficient lubrication.

Meanwhile, in the case of a typical azimuth thruster system, a propeller is rotatable within a range of 360 degrees to enable free forward and rearward propulsion or rotation of a ship. For example, azimuth thrusters, azipods, and the like are used in the azimuth thruster system. The azimuth thruster system is used in various ships including drill ships, icebreakers, shuttle tankers, floating production storage and offloading (FPSO) vessels, polar sailing cargo ships, passenger ships, and the like, owing to control performance and various other advantages.

However, in the case of applying a propulsion method of the above-described counter rotating propeller type propulsion device to the typical azimuth thruster system, the same problems as those of the typical counter rotating propeller type propulsion device may occur, and there is a need for a more effective counter rotating propeller type propulsion device.

DISCLOSURE Technical Problem

It is an embodiment of the present invention to provide a ship propulsion device which may realize counter rotation of two propellers even without an outer shaft and a ship having the same.

In addition, it is another embodiment of the present invention to provide a ship propulsion device which applies a propulsion method to enable counter rotation of two propellers without an outer shaft to an azimuth propulsion method and a ship having the same.

Technical Solution

In accordance with one aspect of the present invention, a ship propulsion device includes a rear propeller fixed to a first drive shaft, a front propeller rotatably supported by the first drive shaft in front of the rear propeller, a counter rotation unit configured to cause counter rotation of the front propeller and the rear propeller based on rotation of a second drive shaft which penetrates a hull and is installed perpendicular to the first drive shaft, and a housing configured to surround the second drive shaft and the counter rotation unit.

The counter rotation unit may include a driving bevel gear fixed to the second drive shaft, a first driven bevel gear fixed to a hub of the front propeller, and a second driven bevel gear fixed to the first drive shaft, and the counter rotation unit may transmit rotation of the driving bevel gear to the first driven bevel gear and the second driven bevel gear to cause counter rotation of the front propeller and the rear propeller.

The counter rotation unit may further include an intermediate bevel gear tooth-engaged between the first driven bevel gear and the second driven bevel gear.

The counter rotation unit may further include a shaft of an intermediate bevel gear, the shaft extending in a direction crossing the first drive shaft to support the intermediate bevel gear.

A bearing may be provided between the intermediate bevel gear and the intermediate bevel gear shaft supporting the intermediate bevel gear for smooth rotation of the intermediate bevel gear.

A first cylindrical lining attached to a front surface of a hub of the front propeller for sealing between the hub of the front propeller and a rear surface of the housing surrounding the second drive shaft, and a first cylindrical sealing member installed to the rear surface of the housing so as to come into contact with an outer surface of the first lining may further be provided.

A second cylindrical lining attached to a front surface of a hub of the rear propeller for sealing between the hub of the rear propeller and a hub of the front propeller, and a second cylindrical sealing member installed to a rear surface of the front propeller so as to come into contact with an outer surface of the second lining may further be provided.

In accordance with another aspect of the present invention, a ship propulsion device rotatably installed to a hull includes a first drive shaft, a rear propeller fixed to the first drive shaft, a front propeller rotatably supported by the first drive shaft in front of the rear propeller, a second drive shaft extending from the hull, the second drive shaft being installed perpendicular to the first drive shaft, and a counter rotation unit configured to transmit rotational force of the second drive shaft to the front propeller and the first drive shaft so as to enable reversed rotation of the front propeller and the first drive shaft.

A housing installed to surround the second drive shaft and the counter rotation unit, the housing being configured to rotatably support the first drive shaft may further be provided.

A first cylindrical lining attached to a front surface of a hub of the front propeller for sealing between the hub of the front propeller and a rear surface of the housing surrounding the second drive shaft, a first cylindrical sealing member installed to the rear surface of the housing so as to come into contact with an outer surface of the first lining, a second cylindrical lining attached to a front surface of a hub of the rear propeller for sealing between the hub of the rear propeller and the hub of the front propeller, and a second cylindrical sealing member installed to a rear surface of the front propeller so as to come into contact with an outer surface of the second lining may further be provided.

The counter rotation unit may include a driving bevel gear fixed to the second drive shaft, a first driven bevel gear fixed to the hub of the front propeller, and a second driven bevel gear fixed to the first drive shaft, and the counter rotation unit may transmit rotation of the driving bevel gear to the first driven bevel gear and the second driven bevel gear to cause counter rotation of the front propeller and the rear propeller.

The first driven bevel gear may be directly connected to the front propeller.

In accordance with a further aspect of the present invention, a ship including a ship propulsion device is provided.

Advantageous Effects

A ship propulsion device and a ship having the same according to the embodiment of the present invention may realize counter rotation of two propellers without an outer shaft.

Further, applying a propulsion method that enables counter rotation of two propellers without the outer shaft to an azimuth propulsion method may enhance propulsion efficiency.

Furthermore, owing to absence of the outer shaft, installation of a drive shaft as well as center alignment of the installed drive shaft may be easily implemented.

In addition, absence of the outer shaft may reduce a required lubrication area than the related art and minimize problems due to lubrication.

DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view showing a propulsion device applied to a ship according to an embodiment of the present invention.

FIG. 2 is a sectional view of the propulsion device according to the embodiment of the present invention.

FIG. 3 is a sectional view of a first sealing unit of the propulsion device according to the embodiment of the present invention.

FIG. 4 is an exploded perspective view of the first sealing unit of the propulsion device according to the embodiment of the present invention.

FIG. 5 is a sectional view of a second sealing unit of the propulsion device according to the embodiment of the present invention.

BEST MODE

The exemplary embodiments of the present invention will hereinafter be described in detail with reference to the accompanying drawings.

As exemplarily shown in FIG. 1, the propulsion device according to the embodiment of the present invention is a counter rotating propeller type propulsion device which generates propulsive force via counter rotation of two propellers 20 and 30. Here, the propulsion device causes counter rotation of the two propellers 20 and 30 based on rotation of a second drive shaft 10 a which penetrates a tail 3 of a hull 1 and is installed perpendicular to a first drive shaft 10. In this case, a drive source 140 (e.g., motor, generator, or engine) to rotate the second drive shaft 10 a is provided within the hull 1. The propulsion device may include a steering unit 150 within the hull 1 to change the direction of propulsive force applied to the hull 1 by the front propeller 30 and the rear propeller 20 to all directions (360 degrees). In addition, the propulsion device may enhance propulsion efficiency using a duct 40 installed to surround the propellers 20 and 30. The duct 40 may have a hydrodynamic streamlined shape.

As exemplarily shown in FIGS. 1 and 2, the propulsion device according to the embodiment of the present invention includes the rear propeller 20 fixed to the first drive shaft 10, the front propeller 30 rotatably supported by the first drive shaft 10 in front of the rear propeller 20, a counter rotation unit 70 to cause counter rotation of the front propeller 30 and the rear propeller 20 based on rotation of the second drive shaft 10 a which penetrates the tail 3 of the hull 1 and is installed perpendicular to the first drive shaft 10, and a housing 130 installed to surround the second drive shaft 10 a and the counter rotation unit 70.

The first drive shaft 10, as exemplarily shown in FIG. 2, is provided with a bearing 139 at a front end of the first drive shaft 10 that is supported in front of the housing 130 for smooth rotation of the first drive shaft 10. In addition, the first drive shaft 10 has a multi-stepped outer surface for sequential installation of the counter rotation unit 70, the front propeller 30, and the rear propeller 20 thereon. The first drive shaft includes a flange portion 11 having a first stepped portion 12 where the counter rotation unit 70 is disposed, and a second stepped portion 13 at the rear of the flange portion 11 for installation of the front propeller 30, the second stepped portion having a smaller outer diameter than that of the first stepped portion 12. In addition, the first drive shaft includes a tapered portion 14 at the rear of the second stepped portion 13 for installation of the rear propeller 20, an outer diameter of which is gradually reduced rearward. The flange portion 11 may be integrated with the first drive shaft 10, or may be prefabricated and then fixed to an outer surface of the first drive shaft 10 via press fitting.

The rear propeller 20 includes a hub 21 fixed to a tail portion of the first drive shaft 10 and a plurality of blades 22 arranged on an outer surface of the hub 21. The rear propeller 20 is fixed to the first drive shaft 10 as an outer surface of the tapered portion 14 of the first drive shaft 10 is press-fitted into a center shaft-coupling bore 23 of the hub 21. In addition, the rear propeller is more firmly fixed to the first drive shaft 10 as a fixing nut 24 is fastened to a rear end of the first drive shaft 10. To achieve this coupling, the shaft-coupling bore 23 of the hub 21 may have a shape corresponding to the outer surface of the tapered portion 14 of the first drive shaft 10. In FIG. 2, reference numeral 25 designates a propeller cap that is mounted to the rear propeller hub 21 to cover the rear end of the first drive shaft 10 and a rear surface of the rear propeller hub 21.

The front propeller 30 is rotatably coupled to the outer surface of the first drive shaft 10 at a position forwardly spaced apart from the rear propeller 20. The front propeller 30 includes a hub 31 rotatably supported by the outer surface of the first drive shaft 10 and a plurality of blades 32 arranged on an outer surface of the hub 31. The front propeller 30 and the rear propeller 20 are configured to implement counter rotation, and therefore blade angles of the front and rear propellers are opposite to each other.

The hub 31 of the front propeller 30 is rotatably supported at the center thereof by a radial bearing 51, and is rotatably supported at both sides thereof by a front thrust bearing 52 and a rear thrust bearing 53 respectively.

The front thrust bearing 52 has an inner race supported by an edge of the second stepped portion 13 of the first drive shaft 10 and an outer race supported by a front bearing support portion 33 of the hub 31. The rear thrust bearing 53 has an inner race supported by a support ring 60 so as not to be axially pushed, the support ring being mounted on the outer surface of the first drive shaft 10, and an outer race supported by a rear bearing support portion 34 of the hub 31. In this case, the radial bearing 51 serves to bear radial load of the front propeller 30 applied in a radial direction of the first drive shaft 10, and the front and rear thrust bearings 52 and 53 serve to bear thrust load applied to the first drive shaft 10 in both axial front and rear directions. In particular, the front thrust bearing 52 serves to bear thrust load applied from the front propeller 30 to the bow during forward movement of the ship, and the rear thrust bearing 53 serves to bear thrust load applied from the front propeller 30 to the stern during rearward movement of the ship.

The hub 31 of the front propeller 30 may be provided with reinforcing members 41 and 42 respectively at positions where the front and rear bearing support portions 33 and 34 are provided. Providing the reinforcing members 41 and 42 respectively at installation positions of the front thrust bearing 52 and the rear thrust bearing 53 increases rigidity of the hub 31. The reinforcing members 41 and 42 may be formed of steel that is more rigid than the hub 31. In the same manner, a reinforcing member 43 may further be provided at a front surface of the hub 21 of the rear propeller 20 at a portion thereof to come into contact with the support ring 60.

Here, after the front propeller 30 and the rear thrust bearing 53 are mounted to the first drive shaft 10, the hub 21 of the rear propeller 20 may be coupled to the first drive shaft 10 via press fitting, and then the support ring 60 may be interposed between the rear propeller hub 21 and the rear thrust bearing 53. The reason why the support ring 60 is installed as described above is because accurately maintaining a distance between the rear thrust bearing 53 and the rear propeller hub 21 is difficult due to a coupling error of the rear propeller caused according to circumstances when the rear propeller 20 is press-fitted to the first drive shaft 10. Accordingly, after the rear propeller 20 is first assembled, a distance between the rear thrust bearing 53 and the rear propeller hub 21 is measured, and the support ring 60 is fabricated to correspond to the distance. In this way, accurate coupling of the support ring and the first drive shaft 10 may be achieved.

The counter rotation unit 70, as exemplarily shown in FIG. 2, causes counter rotation of the front propeller 30 and the rear propeller 20 based on rotation of the second drive shaft 10 a which penetrates the tail 3 of the hull 1 and is installed perpendicular to the first drive shaft 10. In this case, a bearing 138 may be provided between the second drive shaft 10 a and the housing 130 surrounding the second drive shaft 10 a for smooth rotation of the second drive shaft 10 a.

The counter rotation unit 70 includes a driving bevel gear 73 fixed to the second drive shaft 10 a, a first driven bevel gear 71 fixed to the front propeller hub 31, and a second driven bevel gear 72 fixed to the first drive shaft 10. The counter rotation unit 70 transmits rotation of the driving bevel gear 73 to the first driven bevel gear 71 and the second driven bevel gear 72, thereby causing counter rotation of the front propeller 30 and the rear propeller 20.

Here, the driving bevel gear 73, which is fixed to the second drive shaft 10 a extending perpendicular to the first drive shaft 10, is tooth-engaged between the first driven bevel gear 71 and the second driven bevel gear 72.

The first driven bevel gear 71 is secured to the hub 31 as a plurality of fixing bolts 71 a is fastened to the first driven bevel gear in a state in which a rear surface of the first driven bevel gear comes into contact with the front propeller hub 31. In addition, an inner diameter portion of the first driven bevel gear 71 is spaced apart from the outer surface of the first drive shaft 10 to prevent friction during rotation. Although FIG. 2 shows a coupling method of the first driven bevel gear 71 using the fixing bolts 71 a, the first driven bevel gear 71 may be welded to the front propeller hub 31, or may be integrated with the front propeller hub 31.

The second driven bevel gear 72 is secured to the first drive shaft 10 so as to face the first driven bevel gear 71 secured to the front propeller hub 31, and is fixed to the flange portion 11 as a plurality of fixing bolts 72 a is fastened to the second driven bevel gear supported by the first stepped portion 12 of the flange portion 11.

The counter rotation unit 70, as exemplarily shown in FIG. 2, includes an intermediate bevel gear 74 tooth-engaged between the first driven bevel gear 71 and the second driven bevel gear 72. In addition, the counter rotation unit 70 includes an intermediate bevel gear shaft 75 which extends in a direction crossing the first drive shaft 10 to support the intermediate bevel gear 74. A bearing 74 a may be provided between the intermediate bevel gear 74 and the intermediate bevel gear shaft 75 supporting the intermediate bevel gear 74 for smooth rotation of the intermediate bevel gear 74.

The above-described counter rotation unit 70 causes counter rotation of the front propeller 30 and the rear propeller 20 via the plurality of bevel gears 71 to 74, thus having a smaller volume than that of a typical planetary gear type counter rotation unit. In particular, according to the present embodiment, upon installation of the counter rotation unit 70, a rear surface of the first driven bevel gear 71 may face a front surface of the front propeller hub 31 and rotation centers of the first driven bevel gear 71 and the hub 31 may coincide with each other, which enables direct connection between the first driven bevel gear 71 and the front propeller 30. Accordingly, differently from the related art, it is possible to transmit power to the front propeller 30 without using an outer shaft.

Moreover, absence of the outer shaft may ensure less friction of the first drive shaft 10 than the related art, and consequently, ensure a smaller lubrication area than the related art. In addition, absence of the outer shaft may facilitate installation of the first drive shaft 10 and center alignment of the shaft after installation thereof.

A typical planetary gear type counter rotation unit includes a sun gear installed to a drive shaft, a planetary gear around the sun gear, and a cylindrical internal gear around the planetary gear, thus having a relatively large volume. In addition, the planetary gear type counter rotation unit should have a very large volume in consideration of a casing around the internal gear because the internal gear located at an outermost position needs to rotate. In addition, it is necessary to use a hollow shaft corresponding to the typical outer shaft for power transmission from the cylindrical internal gear to the front propeller. In conclusion, the related art has difficulty in achieving a simplified configuration and reduced volume as proposed in the present embodiment.

The propulsion device of the present embodiment, as exemplarily shown in FIG. 2, includes a first sealing unit 90 that seals a gap between a rear surface of the housing 130 and the front propeller hub 31 to prevent invasion of saltwater (or fresh water) or foreign substances, and a second sealing unit 110 that seals a gap between the front propeller hub 31 and the rear propeller hub 21 for the same purpose.

The first sealing unit 90, as exemplarily shown in FIG. 3, includes a first cylindrical lining 91 attached to a front surface of the front propeller hub 31, and a first cylindrical sealing member 92 configured to cover an outer surface of the first lining 91 so as to come into contact with the outer surface of the first lining 91, one end of the first sealing member 92 being secured to the hull tail 3.

The first sealing member 92 includes a plurality of packings 93 a, 93 b, and 93 c arranged at an interval on an inner surface thereof facing the first lining 91 so as to come into contact with an outer surface of the first lining 91, and a path 95 configured to supply fluid for sealing into grooves between the packings 93 a, 93 b, and 93 c. The path 95 of the first sealing member 92 may be connected to a lubricant supply path 137 defined between the second drive shaft 10 a of FIG. 2 and the housing 130 surrounding the second drive shaft 10 a through a connection path 96 to supply lubricant having a predetermined pressure. The lubricant having a predetermined pressure is supplied into the grooves between the packings 93 a, 93 b, and 93 c to press the respective packings 93 a, 93 b, and 93 c onto the first lining 91 until the packings come into close contact with the first lining, which may prevent invasion of saltwater or foreign substances.

The first lining 91, as exemplarily shown in FIG. 4, may include semicircular divided members, i.e. a first member 91 a and a second member 91 b, and thus may be mounted to the first drive shaft 10 after the front propeller 30 is installed to the drive shaft. In addition, a packing 91 d may be provided at a divided portion 91 c of any one of the first and second members 91 a and 91 b to achieve sealing upon coupling of the first and second members.

A free end of the divided portion 91 c of the first member 91 a is provided with a first coupling portion 91 e that protrudes toward the second member, and the second member 91 b is provided with a second coupling portion 91 f corresponding to the first coupling portion for insertion of the first coupling portion. As a fixing bolt 91 g is fastened through the first coupling portion and the second coupling portion, strong mutual coupling of the first and second members is accomplished. A plurality of fixing bolts 91 i may be fastened to a flange portion 91 h fixed to the front propeller hub 31 to achieve strong fixing of the flange portion with respect to the hub 31.

In the case of the first sealing member 92, a plurality of semicircular rings 92 a, 92 b, and 92 c may be stacked one above another in a longitudinal direction of the first drive shaft 10 at the outside of the first lining 91 and fixed to one another. In this case, the plurality of rings 92 a, 92 b, and 92 c may be coupled to one another via bolting or welding.

The second sealing unit 110, as exemplarily shown in FIG. 5, includes a second cylindrical lining 111 attached to a front surface of the rear propeller hub 21, and a second cylindrical sealing member 112 configured to cover an outer surface of the second lining 111 so as to come into contact with the outer surface of the second lining 111, one end of the second sealing member 112 being fixed to a rear surface of the front propeller hub 31. In the same manner as the first sealing member 92, the second sealing member 112 includes a plurality of packings 113 a, 113 b, and 113 c arranged at an inner surface thereof and a path 115 configured to supply fluid into grooves between the packings.

The path 115 of the second sealing member 112 may be connected to a lubricant supply path 137 defined between the second drive shaft 10 a and the housing 130 surrounding the second drive shaft 10 a through a connection path 124. To this end, the first drive shaft 10 and the support ring 60 may be provided with a first radial connection path 121 that connects the lubricant supply path 137 to a space 122 inside the second lining 111. The reinforcing member 42 at the rear surface of the front propeller hub 31 may be provided with a second connection path 123 that connects the space 122 inside the second lining 111 to the path 115 of the second sealing member 112. Lubricant for sealing is supplied from the center of the first drive shaft 10 to the second sealing member 112 to press the packings 113 a, 113 b, and 113 c, which may realize sealing.

Similar to the first lining 91 and the first sealing member 92 of the first sealing unit 90, the second lining 111 and the second sealing member 112 have a semicircular shape so as to be coupled to each other after installation of the rear propeller 20 and the support ring 60.

Next, operation of the propulsion device according to the present embodiment will be described.

In operation of the propulsion device, if the second drive shaft 10 a is rotated via operation of the drive source 140, the driving bevel gear 73 fixed to the second drive shaft 10 a is rotated in the same direction as that of the second drive shaft 10 a. Simultaneously, rotation of the driving bevel gear 73 is transmitted to the first driven bevel gear 71 secured to the front propeller hub 31 and the second driven bevel gear 72 secured to the first drive shaft 10. In this case, the front propeller 30 and the rear propeller 20 implement counter rotation via rotation of the first driven bevel gear 71 and the second driven bevel gear 72.

The front propeller 30 and the rear propeller 20, which implement counter rotation, have blade angles opposite to each other, and therefore generate propulsive water streams in the same direction. That is, the front and rear propellers generate rearward propulsive water streams during forward movement of the ship, and generate forward propulsive water streams during rearward movement of the ship via counter rotation thereof. In addition, with regard to the propulsive water streams generated during forward movement of the ship, the rear propeller 20 acquires propulsive force from rotational energy of fluid having passed through the front propeller 30 via reverse rotation thereof, which results in enhanced propulsion performance. This is equally applied even during rearward movement of the ship. In addition, the steering unit 150 may be used to change the direction of propulsive force applied to the hull 1 by the front propeller 30 and the rear propeller 20, which may change a movement direction of the ship.

Meanwhile, the front propeller 30 generates rearward propulsive water streams during forward movement of the ship, and thus is affected by corresponding repulsive force. This force is transmitted to the first drive shaft 10 via the front thrust bearing 52, thereby serving as propulsive force. Similarly, the rear propeller 20 generates rearward propulsive water streams during forward movement of the ship and is affected by repulsive force. This force is similarly transmitted to the first drive shaft 10 directly connected to the rear propeller, thereby serving as propulsive force.

During rearward movement of the ship, propulsive force (repulsive force) of the front propeller 30 is transmitted to the first drive shaft 10 via the rear thrust bearing 53, and propulsive force of the rear propeller 20 is also transmitted to the first drive shaft 10 directly connected to the rear propeller. In conclusion, the propulsion device of the present embodiment allows propulsive force generated via operation of the front propeller 30 and the rear propeller 20 during forward movement and rearward movement of the ship to be wholly transmitted to the hull 1 through the first drive shaft 10. 

1. A ship propulsion device comprising: a rear propeller fixed to a first drive shaft; a front propeller rotatably supported by the first drive shaft in front of the rear propeller; a counter rotation unit configured to cause counter rotation of the front propeller and the rear propeller based on rotation of a second drive shaft which penetrates a hull and is installed perpendicular to the first drive shaft; and a housing configured to surround the second drive shaft and the counter rotation unit.
 2. The device according to claim 1, wherein the counter rotation unit includes a driving bevel gear fixed to the second drive shaft, a first driven bevel gear fixed to a hub of the front propeller, and a second driven bevel gear fixed to the first drive shaft, and wherein the counter rotation unit transmits rotation of the driving bevel gear to the first driven bevel gear and the second driven bevel gear to cause counter rotation of the front propeller and the rear propeller.
 3. The device according to claim 2, wherein the counter rotation unit further includes an intermediate bevel gear tooth-engaged between the first driven bevel gear and the second driven bevel gear.
 4. The device according to claim 3, wherein the counter rotation unit further includes a shaft of an intermediate bevel gear, the shaft extending in a direction crossing the first drive shaft to support the intermediate bevel gear.
 5. The device according to claim 4, wherein a bearing is provided between the intermediate bevel gear and the intermediate bevel gear shaft supporting the intermediate bevel gear for smooth rotation of the intermediate bevel gear.
 6. The device according to claim 1, further comprising: a first cylindrical lining attached to a front surface of a hub of the front propeller for sealing between the hub of the front propeller and a rear surface of the housing surrounding the second drive shaft; and a first cylindrical sealing member installed to the rear surface of the housing so as to come into contact with an outer surface of the first lining.
 7. The device according to claim 1, further comprising: a second cylindrical lining attached to a front surface of a hub of the rear propeller for sealing between the hub of the rear propeller and a hub of the front propeller; and a second cylindrical sealing member installed to a rear surface of the front propeller so as to come into contact with an outer surface of the second lining.
 8. A ship propulsion device rotatably installed to a hull, the device comprising: a first drive shaft; a rear propeller fixed to the first drive shaft; a front propeller rotatably supported by the first drive shaft in front of the rear propeller; a second drive shaft extending from the hull, the second drive shaft being installed perpendicular to the first drive shaft; and a counter rotation unit configured to transmit rotational force of the second drive shaft to the front propeller and the first drive shaft so as to enable reversed rotation of the front propeller and the first drive shaft.
 9. The device according to claim 8, further comprising a housing installed to surround the second drive shaft and the counter rotation unit, the housing being configured to rotatably support the first drive shaft.
 10. The device according to claim 9, further comprising: a first cylindrical lining attached to a front surface of a hub of the front propeller for sealing between the hub of the front propeller and a rear surface of the housing surrounding the second drive shaft; a first cylindrical sealing member installed to the rear surface of the housing so as to come into contact with an outer surface of the first lining; a second cylindrical lining attached to a front surface of a hub of the rear propeller for sealing between the hub of the rear propeller and the hub of the front propeller; and a second cylindrical sealing member installed to a rear surface of the front propeller so as to come into contact with an outer surface of the second lining.
 11. The device according to claim 8, wherein the counter rotation unit includes a driving bevel gear fixed to the second drive shaft, a first driven bevel gear fixed to the hub of the front propeller, and a second driven bevel gear fixed to the first drive shaft, and wherein the counter rotation unit transmits rotation of the driving bevel gear to the first driven bevel gear and the second driven bevel gear to cause counter rotation of the front propeller and the rear propeller.
 12. A ship including a ship propulsion device according to claim
 1. 